A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus are conventionally used, for example, in the manufacture of integrated circuits (ICs), flat panel displays and other devices involving fine structures.
It is desirable to reduce the size of features in a lithographic pattern because this allows for a greater density of features on a given substrate area. In photolithography, the increased resolution may be achieved by using light of shorter wavelength. However, there are problems associated with such reductions. Current systems are starting to adopt optical sources with wavelengths in the 193 nm regime but even at this level, diffraction limitations become a barrier. At lower wavelengths, the transparency of materials is very poor. Optical lithography machines capable of enhanced resolutions require complex optics and rare materials and are consequently very expensive.
An alternative for printing sub-100 nm features, known as imprint lithography, comprises transferring a pattern to a substrate by imprinting a pattern into an imprintable medium using a physical mould or template. The imprintable medium may be the substrate or a material coated on to a surface of the substrate. The imprintable medium may be functional or may be used as a “mask” to transfer a pattern to an underlying surface. The imprintable medium may for instance be provided as a resist deposited on a substrate such as a semiconductor material to which the pattern defined by the template is to be transferred. Imprint lithography is thus essentially a moulding process on a micrometer or nanometer scale in which the topography of a template defines the patterns created on a substrate. Patterns may be layered as with optical lithography processes so that in principle imprint lithography could be used for such applications as IC manufacture.
The resolution of imprint lithography is limited only by the resolution of the template fabrication process for instance, imprint lithography has been used to produce features in the sub-50 nm range with significantly improved resolution and line edge roughness compared to that achievable with conventional optical lithography processes. In addition, imprint processes do not require expensive optics, advanced illumination sources or specialised resist materials typically required by optical lithography processes.
Current imprint lithography processes do have a number of drawbacks as will be mentioned below, particularly with regard to achieving overlay accuracy and high throughput. However the significant improvements in resolution and line edge roughness attainable from imprint lithography are strong drivers for addressing these and other problems.
A problem which may arise during imprint lithography is that gas bubbles, also known as air inclusions, may be trapped underneath an imprint template during the imprint process. These trapped gas bubbles do not migrate out from beneath the imprint template, but instead stay there for the duration of the imprint process. When the imprint template is removed from the imprintable material (which may be a UV curable resin), an area of the imprinted pattern will be incomplete or otherwise damaged at the location of the gas bubble.